1. Field of the Invention
This invention relates to the field of semiconductor heterojunction devices and, in particular, to transistors, optical emitters, optical detectors, optical modulators, optical amplifiers and other optoelectronic devices utilizing an inversion channel created by modulation doping.
2. State of the Art
This invention builds upon the existing device structure known as the Pseudomorphic Pulsed Doped High Electron Mobility Transistor (Pulsed Doped PHEMT) and sometimes referred to as the Pulsed Doped Modulation Doped Field Effect Transistor (Pulsed Doped MODFET) or the Pulsed Doped Two Dimensional Gas Field Effect Transistor(Pulsed Doped TEGFET). GaAs/InGaAs/AlxGa1−xAs is the III-V material system of choice for these devices because of the ability to grow high optical/electrical quality epitaxial layers by MBE (molecular beam epitaxy). These high frequency transistors are now in constant demand as the front end amplifier in wireless and MMIC applications and they have become well recognized for their superior low noise and high frequency performance.
The use of pulse doping in the HEMT epitaxial structure was first disclosed at the IEEE Cornell conference on high speed devices in Aug. 1983 (Lee 1983), in the context of the GaAs/AIGaAs HEMT device. In that case the heterojunction interface containing the inversion channel was formed between GaAs and AlGaAs materials. In a later publication (Rosenberg 1985), a strained layer of InGaAs was employed at the heterojunction with GaAs both above and below the quantum well. Then in 1987, Morkoc and coworkers patented the Pseudomorphic HEMT structure, which is the structure reported by Rosenberg but with the GaAs above the quantum well replaced by AlxGa1−xAs.
The pseudomorphic transistor structure has been very successful in producing microwave transistors that operate well into the multi-gigahertz regime, initially being used extensively in military systems and now finding their way into commercial products, particularly in the area of cellular communications. There has been a growing interest in combining the PHEMT with optical capability because of the difficulty in propagating very high frequency signals to and from the integrated circuit by coaxial lines. Combining electronic with optoelectronic components monolithically gives rise to the concept of the optoelectronic integrated circuit (OEIC). However, there are serious problems encountered because of the dissimilar nature of the structures of the FET, the pn junction laser and the MSM or PIN diode. To achieve this goal it has been proposed to change the structure by modifying the growth between the quantum well and the interface to enable an ohmic contact instead of a Schottky contact (see U.S. Pat. No. 4,800,415). In this patent, the PHEMT growth structure is modified in the region between the modulation doping and the semiconductor surface and the doping is proposed to be substantially p-type in order to provide a low resistance ohmic contact for the gate of the FET. However, this high doping creates a problem in the formation of the vertical cavity laser because of the effects of free carrier absorption. It also creates a problem in forming depletion-type FETs by implanting n-type dopant, i.e., compensating a large p density with a large n density to obtain a lower p density is difficult to control in a bulk region but much easier in a delta doped region. It makes control of the enhancement threshold difficult too, because the input capacitance is a function of doping which is harder to control than layer thickness. Another problem with this doping scheme is in producing effective current funneling for the laser to direct the current flow into the region of stimulated emission. It is very desirable to create a pn junction by N type implantation to steer the current in this structure since this would be compatible with the overall approach to building the FET devices. The heavy p doping makes it difficult to create junction isolation that is low leakage.